1. Field of the Invention
The present invention relates to a device for tightly fixing a coil on a cylindrical body. The present invention is suitable for use with a heating device for a hot sprue or hot runner of an injection moulding system.
2. Description of the Related Art
Known in the prior art is a heating device for a hot sprue or hot runner block in an injection moulding system, the heating device having a cylindrical body with a coiled heater made of sheathed heating wire arranged thereon, and a fixing band arranged around the coiled heater. This fixing band has facing flange portions, and clamping members such as bolts and nuts for connecting the facing flange portions of the band so that the coiled heater is held between the cylindrical body and the band.
The sheathed heating wire is pre-formed into the shape of a coil, the diameter of which is slightly larger than that of the cylindrical body. The coil shaped sheathed heating wire is pushed over and onto the cylindrical body, the band arranged around the coiled heating wire, and the clamping bolts then tightened.
However, this prior art device suffers from a drawback in that, since the diameter of the coil is larger than that of the cylindrical body, it is difficult to completely place the heater into direct contact with the surface of the cylindrical body along the entire length thereof, even if the heater is urged inward and tightened by means of the fixing band. As a result, gaps are generated between the coil heater and the cylindrical body, making it more difficult to transfer all of the heat from the heater to the cylindrical body.
To overcome this difficulty, another prior art device has proposed that the coil shape of the sheathed heater be determined in such a manner that the diameter of the coiled heater is slightly smaller than that of the cylindrical body. In this method, when the coiled heater is arranged on the cylindrical body the coiling must be first loosened to enable the coil to fit over the body. After the fitting of the coil, a fixing band is arranged around the coil in such a manner that the coil is tightened between the fixing band and the cylindrical body by the clamping bolts and nuts. However, this device suffers from a drawback in that the coil must be first loosened to allow the coil to be fitted on the cylinder, which requires extra work, and that a force for tightening the coil heater to place it completely in direct contact with the surface of the cylindrical body still cannot be obtained.